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This is an illustration of the interior of Jupiter’s largest moon Ganymede. It is based on theoretical models, in-situ observations by NASA’s Galileo orbiter, and Hubble Space Telescope observations of the moon’s magnetosphere, which allows for a probe of the moon’s interior. The cake-layering of the moon shows that ices and a saline ocean dominate the outer layers. A denser rock mantle lies deeper in the moon, and finally an iron core beneath that. Credit: NASA, ESA, and A. Feild (STScI)

March 13, 2015 – NASA’s Hubble Space Telescope has the best evidence yet for an underground saltwater ocean on Ganymede, Jupiter’s largest moon. The subterranean ocean is thought to have more water than all the water on Earth’s surface.

Identifying liquid water is crucial in the search for habitable worlds beyond Earth and for the search of life as we know it.

“This discovery marks a significant milestone, highlighting what only Hubble can accomplish,” said John Grunsfeld, associate administrator of NASA’s Science Mission Directorate at NASA Headquarters, Washington. “In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system. A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth.”

Ganymede is the largest moon in our solar system and the only moon with its own magnetic field. The magnetic field causes aurorae, which are ribbons of glowing, hot electrified gas, in regions circling the north and south poles of the moon. Because Ganymede is close to Jupiter, it is also embedded in Jupiter’s magnetic field. When Jupiter’s magnetic field changes, the aurorae on Ganymede also change, “rocking” back and forth.

By watching the rocking motion of the two aurorae, scientists were able to determine that a large amount of saltwater exists beneath Ganymede’s crust affecting its magnetic field.

NASA Hubble Space Telescope images of Ganymede’s auroral belts (colored blue in this illustration) are overlaid on a Galileo orbiter image of the moon. The amount of rocking of the moon’s magnetic field suggests that the moon has a subsurface saltwater ocean. Image Credit: NASA/ESA

A team of scientists led by Joachim Saur of the University of Cologne in Germany came up with the idea of using Hubble to learn more about the inside of the moon.

“I was always brainstorming how we could use a telescope in other ways,” said Saur. “Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior.”

If a saltwater ocean were present, Jupiter’s magnetic field would create a secondary magnetic field in the ocean that would counter Jupiter’s field. This “magnetic friction” would suppress the rocking of the aurorae. This ocean fights Jupiter’s magnetic field so strongly that it reduces the rocking of the aurorae to 2 degrees, instead of the 6 degrees, if the ocean was not present.

Scientists estimate the ocean is 60 miles (100 kilometers) thick – 10 times deeper than Earth’s oceans – and is buried under a 95-mile (150-kilometer) crust of mostly ice.

Scientists first suspected an ocean in Ganymede in the 1970s, based on models of the large moon. NASA’s Galileo mission measured Ganymede’s magnetic field in 2002, providing the first evidence supporting those suspicions. The Galileo spacecraft took brief “snapshot” measurements of the magnetic field in 20-minute intervals, but its observations were too brief to distinctly catch the cyclical rocking of the ocean’s secondary magnetic field.

The new observations were done in ultraviolet light and could only be accomplished with a space telescope high above the Earth’s atmosphere, which blocks most ultraviolet light.

Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington.

Ball Aerospace & Technologies Corporation of Boulder, Colorado plays a major role in the success of NASA’s Hubble Space Telescope. All of the instruments currently on board the Hubble Space Telescope are built by Ball Aerospace. The Cosmic Origins Spectrograph (COS) was a joint project of the University of Colorado at Boulder, the University of California – Berkley, and Ball Aerospace.

Hubble is currently celebrating its 25th Anniversary in space on April 24. Hubble has made some of the most dramatic discoveries in the history of astronomy. From its vantage point 600 km above the Earth, Hubble can detect light with ‘eyes’ 5 times sharper than the best ground-based telescopes and looks deep into space to unravel the mysteries of the universe.

Caption: This is a video clip of what Ganymede looks like, based on images from NASA’s Galileo orbiter. The US Geological Survey has classified the surface of Ganymede into the types of terrain.

The brown regions are those that are heavily cratered and much older than the light shaded regions that are smoother with few craters.

These lighter shaded regions are believed to be formed by flooding of the surface with water coming from faults or even cryo-volcanos that have taken place over billions of years. Perhaps even tectonic processes are at work with some crustal ice sheets being forced downward by the emergence of newer icy material.

The Galileo spacecraft made six close flybys of the Ganymede and detected a magnetic field coming from the moon itself. In addition, the best models of Ganymede from the Galileo data showed a deep ocean under a thick ice crust. Credit: NASA, USGS